Pin tumbler locks comprise a housing carrying a cylindrical plug. Rotation of the plug within the housing activates the cam mechanism by which a locking bolt is manipulated, unlocking the lock. The plug has a series of holes, each to accommodate a spring-loaded pin having top (driver pin) and bottom (key pin) portions of varying length. When the pins are correctly positioned by inserting the correct key in the keyway, the top portions of all of the pins are in the housing and the lower portions are in the plug, aligning the gap between them with the shear line and allowing the plug to rotate in the housing along the shear line. In theory, the plug rotates only when the correct key is inserted; otherwise the bodies of the pins physically prevent such rotation by blocking the shear line.
However, pin tumbler locks have a major flaw that makes them vulnerable to picking: they allow a user to move the pins and spin the plug at the same time, even without the proper key. If the user is able to spin the plug enough, for example using a tension wrench, it can create a ledge upon which the driver pins can be rested. That is, each pin can be manipulated to move its top portion above the shear line and to rest it on the ledge, while the lower portions move into the plug. Once all of the pins are out of the way of the shear line, the lock is picked.
Bumping is another way in which pin tumbler locks may be opened without the correct key. This technique requires a bump key to be inserted into the keyway. The bump key is then tapped lightly, briefly separating the key and driver pins so the driver pins are all above the shear line. This allows the plug to rotate, if the user turns the key quickly while the pins are still out of the way.
Earlier lock designs have tried to make picking more difficult by creating things like mushroomed pins and multiple sets of pins. For example, U.S. Pat. No. 6,584,819 discloses a lock having two sets of pins, each operated by a different area of the same key. The key must be inserted and then rotated to ensure that both a flat portion of the key and a cylindrical portion of the key connect with the appropriate areas of the lock body. International Pub. No. WO 91/14064 discloses a cylinder lock in which each pin is divided into at least four pieces, providing two shear gaps, each of which must be properly aligned before the cylinder will rotate. However, such designs generally fail to properly address the root shortcoming with pin tumbler locks, namely the ability to spin the plug and move the pins simultaneously. U.S. Pat. No. 2,281,714 discloses an anti-bumping lock comprising a resilient layer around the pins, such that they are not easily moved except along a perfectly straight axis, i.e. by insertion of the proper key, which may be difficult to manufacture and to maintain.
Other lock designs have tried to secure the lock through additional mechanisms such as a secondary protrusion from the core into the housing, preventing the lock core from rotating until that protrusion is retracted, generally by insertion of a correctly cut key. For example, DE 102010 035809 discloses a secondary mechanism comprising balls that abut a recess in the inner housing of the lock, such that the lock core cannot rotate unless the correct key moves a shear pin out of the way, allowing the balls to move away from the housing. DE 10208893, U.S. Pat. Nos. 1,965,889, 3,837,197, 6,481,255, 9,435,138 and 9,482,031, and many others comprise one or more locking bars that interact with the housing and prevents cylinder movement until they are properly retracted. U.S. Pat. No. 5,488,847 comprises a pawl that can only be moved out of the way by a first rotation of the key, followed by a rapid rotation of the key in the opposite direction. However, these additional mechanisms generally complicate the overall design and manufacture of the lock, and still fail to address the primary shortcomings.
Still other lock designs have several interacting pins, plates or other components within the lock core to increase the difficulty of picking the lock. Examples of these types of locks include GB 2266918, U.S. Pat. Nos. 6,481,255 and 6,584,819 and U.S. Pub. No. 2006/0230797. However, these designs require specially shaped keys which are difficult and costly both to manufacture and to replace if lost. In addition, the inner workings of the lock are often extremely complex and require precision machining, making the lock difficult and expensive to manufacture to required standards.
It is therefore an object of this invention to provide a pick-resistant lock that overcomes the foregoing deficiencies.
It is a further object of the invention to provide a pick-resistant lock that restricts the rotational movement of the plug while the pins are able to be moved.
It is a further object of the invention to provide a pick-resistant lock that restricts movement of the pins while the plug is able to be rotated.
These and other objects of the invention will be better understood by reference to the detailed description of the preferred embodiment which follows. Note that the objects referred to above are statements of what motivated the invention rather than promises. Not all of the objects are necessarily met by all embodiments of the invention described below or by the invention defined by each of the claims.